Conventionally, a coding technique of Moving Picture Expert Group (MPEG) (see Non-Patent Reference 1) has been used for compress-coding (hereinafter simply referred to as “coding”) moving picture data using inter-frame differences. Further, in recent years, H.264 that achieves higher compression (see Non-Patent Reference 2) has been adopted as a coding standard.
Inter-frame prediction, an elemental technique for coding and decoding moving picture data, requires a memory for storing decoded pictures because pixel values are predicted using information on the decoded pictures. The memory has conventionally been provided outside a system LSI that includes the decoding unit and so on, in view of a necessary storage capacity and a chip area (see Patent Reference 1).
Pictures stored in the external memory include a currently decoded picture, a picture which may be referred to by the currently decoded picture, and a picture outputted to a display unit.
Thus, it is necessary to perform processing such as sequentially reading decoded pictures from the external memory to the decoding unit for inter-frame prediction and so on, as well as writing a currently decoded picture to the external memory.
Particularly, the processing of sequentially reading decoded pictures for inter-frame prediction involves frequent data transfer and a large transfer volume. This has resulted in a problem of high cost because the access bandwidth of the external memory is increased and a high-speed memory is required as the external memory.
To address this problem, a high-capacity memory with an embedded system LSI (Embedded DRAM) is now in practical use, and a technique for reducing external memory access by utilizing an internal memory as well as the external memory is being developed.
Patent Reference 2 discloses a technique of enhancing processing efficiency of motion estimation in the coding processing of MPEG by writing data used for motion estimation to both the internal and external memories, and placing necessary data in the wide-band internal memory (see Patent Reference 2).
Patent Reference 1: Japanese Unexamined Patent Application Publication No. 2004-215049
Patent Reference 2: Japanese Unexamined Patent Application Publication No. 2002-218474
Non-Patent Reference 1: ITU-T recommendation H.262 “Information technology—Generic coding of moving pictures and associated audio information: Video”
Non-Patent Reference 2: ITU-T recommendation H.264 “Advanced video coding for generic audiovisual services”